Some approximate relations in the photoelasticity of strongly-anisotropic crystals

The knowledge of the principal refractive indeces and their dependence on the applied stress are the starting point for any experimental analysis of transparent crys- tals. Since for strongly-anisotropic crystals and general state of stress an explicit analytical solution can be difficult to obtain, then we need some approximated solu- tions. Here we propose a coherent approximation procedure which generalizes results already obtained and give a simple example of application.